1. Field of the Invention
This invention relates generally to lens systems and more specifically to a lens system suitable for use as an objective lens and using elements which exhibit axial gradient index of refraction.
2. Description of Prior Art
U.S. Pat. Nos. 4,883,522; 4,907,864; 4,929,065; and 5,200,858 each to Hagerty et al. disclose gradient optical density transmission of light directing devices and fabrication thereof. For instance, U.S. Pat. No. 4,929,065 discloses that a glass block comprises a plurality of intermediate glass plates stacked between a top plate and a bottom plate. The top and bottom plates each have a selected composition, with a particular index of refraction and a softening temperature. The intermediate plates have compositions intermediate those of the top and bottom plates and are stacked so as to provide a gradient in composition from top to bottom. This assembly is heated to fuse the plates together thus resulting in a single lens element which has a gradient in its refractive index in the range of for instance 0.085 to 0.5 or greater. This axial gradient index lens element provides aberration correction and design simplifications over conventional lenses including aspheric lenses. This process allows production of gradient index (GRIN) lens elements of relatively large diameter. Such lenses are especially suitable for replacement of conventional molded glass aspheric lenses, which are expensive to manufacture in large diameter and to close optical tolerances.
Axial GRIN lens elements have refractive index profiles that vary along a given axis (typically the optical axis). The axial index contours are thus planes perpendicular to the optical axis. The gradient of the index of refraction need not be linear, but has been found by most lens designers that it is easier to work with linear axial gradients for purposes of analysis. Although the index of refraction of GRIN lens elements is directly controlled, the dispersion characteristics (Abbe number) of the GRIN material is not so controlled.
"Design of a 35-mm Photographic Objective Using Axial GRIN Materials" by Richard N. Pfisterer, SPIE Volume 2000, July 1993, discloses design of a photographic objective lens system employing axial GRIN elements. The lens system has four lens elements with symmetry about a central aperture stop and has two GRIN lens elements, these being the lens elements closest to the object and closest to the image. There is a very small amount of residual oblique spherical aberration. The performance was found to be superior to any configuration using aspheric lens surfaces but without GRIN lens elements. That is, the GRIN elements improved chromatic aberration correction over conventional aspherical lens elements.
Due to the relative recent advent of such large diameter GRIN lens elements, their application has not been wide spread, and use to date of such lenses has concentrated on their index of refraction characteristics without regard to other characteristics. For instance, the above-mentioned Pfisterer paper overly emphasizes the role of the GRIN lens element as opposed to the effect of utilizing high index glass materials (without gradient) to correct optical aberration. The Pfisterer paper demonstrates that use and characteristics of GRIN lenses are not well understood. Moreover, prior art GRIN lens systems have emphasized chromatic aberration correction for meniscus lens shapes.